What causes this mist pass glitch?
Multiple transparent planes with low Max Transparecy Bounces

The "Tree1" object is made by multiple overlapping planes. Their shading properties comes out from a sharp Mix of a Transparent shader and a Diffuse shader.

As there are multiple intesection and your Max Transparency Bounces is set to 8, light can't filter trough all the planes.



We can clearly see this by giving to the material a full Transparent shading.



More Bounces leads to the expected shading behaviour:



Now that the shader works as expected, you'll have no issue in Mist pass calculation:





Thought about Mist pass caluculation. I didn't find any evidence of the following lines in documentation, so take it as my (current) opinion, and feel free to discuss about

When Light reaches the max number of bounces stadted by the Light Path settings the simulation is "over". This means that the path tracer cosider the light information in the same way as the light would have reached the background. And as is considered "background", its mist pass value will be the higer possible (1=fullwhite).